The examination and observation of specimens of sub-micron dimensions is of great interest to scientists and engineers doing research in the physical and biological sciences. It is also of great scientific value to study the effects of experimental treatments on such specimens and to examine any changes, modifications, transformations, and other effects that result from experimental treatments of these specimens. The specimens of interest are of extremely small dimensions and they can only be observed in very advanced microscopes such as transmission electron microscopes (TEM), scanning electron microscopes (SEM), atomic force microscopes (AFM), and other electronic or optical microscopes.
The process of finding objects such as point defects (or clusters of point defects), line defects (such as dislocations), planar defects (such as stacking faults), and other volume defects (such as voids) in a specimen requires a high-resolution microscope. The same requirement exists for biological specimens and for other materials science research in industry, including the semiconductor industry. Once these objects of interest are located in a specimen and observed in a microscope, it becomes equally important to study the effects of experimental treatments on the objects. Such study requires that the specimen be removed from the microscope and subjected to different ex-situ treatments such as plasma exposure, heat, or other chemical or physical treatment that cannot be performed on the specimen while it is in the microscope.
Removable specimen holding systems used to retain specimens for observation under microscopes are well known to those of skill in the art. More specifically, systems for holding silicon wafers (or portions of those wafers) having a plurality of semiconductor chips imprinted on the wafers for viewing under electron beam microscopes are well known. Such systems typically include a specimen mounting grid having a specimen mesh upon which the wafer or wafer portion (i.e., the specimen) is mounted. The specimen mounting grid is secured to a specimen holder (or platen) which, in turn, is positioned on the stage of the microscope. The orientations (1) between the specimen and the specimen grid, (2) between the specimen grid and the specimen holder, and (3) between the specimen holder and the microscope stage are all critically important. The prior art has addressed the first and third of these important orientations; the present invention is directed to the second orientation--the orientation between the specimen grid and the specimen holder.
A. Orientation Between Specimen & Specimen Grid
A relatively large number of references address orientation between the specimen and the specimen grid or equivalent structure. U.S. Pat. No. 4,943,148 issued to Mondragon et al. discloses, for example, a holder for positioning silicon wafers on a microscope stage. The device assures desired orientation between the specimen wafer and the structure holding the specimen (i.e., a specimen grid). Semiconductor processing quality control procedures require that each wafer be analyzed at an appropriate number of sites so that representative chips may be viewed to insure proper quality. Such analysis occurs under a SEM, with the sites selected by a computer-controlled stage onto which the wafer is placed.
In order to assure that identical sites are reviewed on successive wafers, the wafer must occupy the same position on the structure used to hold the wafer. The wafer should be positioned on the structure to within close tolerances, on the order of .+-.150 microns or less. Such uniform positioning of the wafer within the holding structure assures accurate and repeatable microscopic views. Calibration standards are built into the holding structure, reducing the time and variability incident to such calibration.
The wafers according to the device taught by Mondragon et al. have a major flat and at least one minor flat. A clamp affixed to a threaded member slides within a radially aligned slot so as to be aligned with the major flat. Therefore, rather than aligning the wafer against a pair of posts, the clamp pushes the leading edge of the wafer against a face, thereby self-aligning the wafer in the same position relative to the holding structure each time a wafer is placed in the holding structure.
Another reference which relies upon marks, scribes, notches, flats, or other structural alterations of the specimen itself to provide orientation is U.S. Pat. No. 5,497,007, issued to Uritsky et al. The Uritsky et al. patent discloses an automated method for determining the wafer coordinates in a SEM/EDX system. Semiconductor wafer characterization equipment typically includes a high-magnification imaging system such as a SEM coupled to an energy dispersive x-ray (EDX) detector. Such an imaging system, when used to scan a semiconductor wafer, provides information regarding particles and anomalies on the surface of the wafer. The combination of a SEM and an EDX within a common unit is generally known as a SEM/EDX unit.
A laser scanner creates a laser scan map of the coordinates of the wafer features and concomitant particles. This laser scan map uses the coordinate system of the laser scanning device, of course, to identify the location of surface features and particles. The manner in which the laser beam is scattered from the wafer surface features and particles yields signals from which estimated particle positions in terms of x and y coordinates can be determined. A computer controls the electron beam intensity, scan rate, and position relative to the coordinate system of the SEM/EDX unit (also the imaging system coordinate system).
Because the wafer is physically moved from the laser scanner to the imaging system, there is no way to guarantee that the coordinate system used in the laser scanner will apply when the wafer is moved to the imaging device. Thus, the wafer coordinate system must be related to the coordinate system of the SEM/EDX unit. The wafer coordinate system is defined by the location of the center of the wafer and the orientation angle of the wafer relative to the imaging system coordinate system. The orientation angle of the wafer is defined by the position of a significant wafer landmark, including a notch or flatted portion on the wafer itself. By relating the imaging coordinate system to the wafer coordinate system, the high-magnification imaging system can repeatedly find any location in the wafer coordinate system.
Typically, the wafer is maintained on the SEM stage by a specimen holder that is roughly the shape of the wafer and a spring-loaded pin (or key) that interacts with the edge of the wafer. The pin maintains the wafer in a stationery position within the specimen holder. An operator roughly aligns the specimen holder with predefined markings on the SEM stage. As such, each wafer that is analyzed is generally oriented in the same direction, i.e., the notch or flatted portion of the wafer points in roughly the same direction relative to the stage.
The method suggested by Uritsky et al. is only useful as long as the specimen stays in the microscope. When the specimen is removed for ex-situ treatment, the re-location of a previously identified defect or other object on the specimen would pose a problem. Uritsky et al. do not address that problem. Thus, there remains a need for improved orientation between the specimen grid and the specimen holder.
The automated method taught by Uritsky et al. permits a wafer to be arbitrarily oriented in the imaging system coordinate system "as long as the general location of the landmark region is known to the imaging system." Column 10, lines 60-63. The landmark region is a notch or flattened portion on the wafer itself. Frequently, the nature of specimens is such that they cannot be "marked" or "scribed" with notches, flats, or other physical characteristics. There also remains a need, therefore, for an orientation system which avoids structural alterations of the specimen itself.
Uritsky et al. address a SEM/EDX system. The orientation system currently available on advanced TEM systems allows specimen orientation only when the specimen is inside the microscope. Orientation is done by tracking the x-y coordinates using a computer and the repeated examination of any event is only possible as long as the specimen remains in the microscope. Such current systems are not very helpful if any defect or other artifact must be examined repeatedly to observe the effects of ex-situ processing or treatment such as plasma exposure. There remains a need, therefore, for a simple orientation system that would allow the tracking of a particular specimen site so that the site might be examined repeatedly after ex-situ processing.
In their article titled "Mask Aligning Fixture For Silicon Wafers," International Business Machines Technical Disclosure Bulletin No. FI882-0962, Vol. 27, No. 4B, pages 2383-84 (September 1984), authors R. Christensen and R. Imrie disclose a technique predicated on the use of a mask material that matches the coefficient of expansion of silicon. Such matching eliminates the need for compensated artwork. Like the devices taught by Mondragon et al. and Uritsky et al., however, the orientation structure disclosed by R. Christensen and R. Imrie requires modification of the wafer.
Specifically, the silicon wafer has fiducials or via holes which allow it to be oriented with respect to a mask. The mask is also marked for alignment purposes, having a "V" notch. The mask is located by two, straight mask registration pins separated by approximately 90 degrees on the periphery of the mask holder. One of the pins engages the V-shaped notch on the mask. The silicon wafer is held by vacuum and manipulated under the mask until the fiducials or the via holes are lined up, as viewed through a microscope. Cams are rotated until they touch the outer edge of the silicon wafer. The cams are locked in place by an impactor tool that wedges tapered pins into a tapered hold. Square holes in the cam that slide over the square shank of the locking pin prevent the cam from turning. A spring holds the mask in registration with the two mask registration pins. A wafer spring holds the silicon wafer against the cams. A wafer/mask spring is centered on the wafer. Then a spring clip top plate is placed over "U" clip posts. The entire assembly is clipped to the mask holder by "U" clips.
Thus, the apparatus and method disclosed by R. Christensen and R. Imrie align the wafer to the mask with the help of fiducials, vias, or other alignment marks on the wafer. The need for an orientation system which avoids structural alterations of the specimen itself has been discussed above. The authors are also not concerned with re-location of previously observed objects or defects on the specimen after ex-situ processing.
The conventional technique of structurally altering the wafer specimen to facilitate orientation is further illustrated by C. Aliotta, A. Constantino & G. Cia, "Holder For Automatic Alignment Of Large Wafers In A Scanning Electron Microscope," International Business Machines Technical Disclosure Bulletin No. YO882-0641, Vol. 26, No. 10B, pages 5479-80 (March 1984). The wafer is retained in a holder by three contact points along the wafer edge. A movable contact pin on the holder is spring-loaded and designed to fit into an alignment slot on the edge of the wafer. This engagement serves two purposes: (1) the slot allows each succeeding wafer to fit on the holder in the same orientation each time, and (2) the spring-loaded pin applies pressure to the stationary hold-down pins. Thus, the wafer is positioned and secured to the holder in one operation.
Typically, it is not possible to self-align a wafer in practice for automatic loading into a microscope. The delicate nature of the specimen often renders impossible the spring loading taught by Aliotta et al. to hold the specimen. Rather, an operator is required to load most specimens. Even with human specimen loading, the relatively bulky spring-loaded holder is not feasible because limited space exists in most microscopes to accommodate such a holder--especially given the need for x-y-z translational and theta rotational movement of the specimen.
Anonymous, "Microscope Wafer Orientation Fixture," International Business Machines, Research Disclosure No. 25815 at page 515 (October 1985), also illustrates the conventional technique of structurally altering the wafer specimen to facilitate orientation. The disclosure teaches a fixture for roughly orienting a semiconductor wafer within the field of view of an optical microscope. The fixture baseplate is mounted on the microscope stage and moves with the stage in the vertical direction of focusing, and in the x and y directions, if the microscope is so provided. The baseplate is provided with two or more sets of adapters for different sizes of wafers. Each set consists of a wafer holder and a template holder. Both the wafer holder and the template holder have pins and mating holes for fixing the wafer and template, relative to each other, both angularly and orthogonally in the x and y directions.
All wafers in the anonymous reference are provided with a chordal flat, a notch, or both structural features for angular orientation. A pointer affixed to the microscope overlies the template to permit a corresponding area of the wafer to be examined as the fixture is translated to a desired area, using the template as a coordinate address reference. The pins are used primarily to lock the wafer holder and template onto the base plate rather than for orientation purposes. The wafer is oriented using the chordal flat or a notch on the wafer itself.
B. Orientation Between Specimen Holder & Microscope
U.S. Pat. No. 4,596,934 issued to Yanaka et al. discloses an electron beam apparatus with an improved specimen holder. The apparatus seeks to reduce spherical aberration in the microscope by decreasing the working distance between the specimen supported on the specimen grid and the objective lens. This is accomplished by modifying the structure of the specimen holder while maintaining adequate rigidity and vibration insensitivity of the specimen holder. As a peripheral result, the modified specimen holder is more easily oriented to the column of the microscope.
Specifically, the specimen holder is provided with a cutout or thin region in the end which is inserted into the microscope. The very edge of the thin region has a notch located at its mid-point. The thin region allows the objective lens of the microscope to be placed closer to the specimen than when a thicker specimen holder is inserted; the thin region permits reduction in the working distance of the microscope. When the specimen holder is positioned in the microscope, the notch of the thin region of the specimen holder engages a wedge-like member fixedly mounted on the inner side wall of the microscope column. Such engagement supports the specimen holder stably despite the structural weakness of the thin region.
Although not discussed by Yanaka et al., the engagement between the specimen holder and the microscope column would also appear to help orient the specimen holder in the microscope. Yanaka et al. clearly do not address, nor is the apparatus disclosed by Yanaka et al. applicable to, the problem of orientation between the specimen grid and the specimen holder. Accordingly, specimen orientation and re-examination of previously detected artifacts or defects on the specimen following ex-situ processing remains an issue both unrecognized and unsolved by Yanaka et al.
The deficiencies of the conventional microscope orientation systems show that a need still exists for a system that facilitates in-situ and ex-situ repeated analyses of a specimen. To overcome the shortcomings of the conventional systems, a new microscope specimen holder and grid arrangement is provided. An object of the present invention is to permit repeated orientation between the specimen holder and the specimen grid, allowing the specimen grid to be removed from the specimen holder and replaced in the specimen holder in precisely the same location following ex-situ processing of the specimen affixed to the specimen grid. Such a system assures easy relocation of a specimen feature after repeated ex-situ and in-situ experiments, treatments, or processes. A related object is to provide a relatively simple orientation system that allows the tracking of a particular specimen site so that the site might be examined repeatedly after ex-situ processing.
An additional object of the present invention is to modify the conventional specimen holder structure while maintaining adequate rigidity and vibration insensitivity of the specimen holder. Yet another object of the invention is to provide a flexible orientation system that permits the specimen to be tilted and translated while in the microscope. Such a system allows the operator to take advantage, without restriction, of all possible horizontal, vertical, and angular motions of the microscope for observation of the specimen while using the orientation system of the present invention. A related object is to provide an orientation system that can be used in all types of microscopes. Finally, it is still another object of the present invention to avoid marks, scribes, notches, flats, or other structural alterations of the specimen itself.